Lithium secondary batteries (typically lithium-ion batteries), nickel-metal hydride batteries, and other secondary batteries have become more important recently as a vehicle-mounted electric power source, as well as for use in personal computers and portable devices. In particular, it is expected that lithium secondary batteries, which are lightweight and can provide high energy density, can be preferably used as a high output electrical power source for vehicles.
A lithium composite oxide is generally used as the positive electrode active material comprising the positive electrode of a lithium secondary battery. A composite oxide comprising lithium, nickel, cobalt, and manganese (hereafter, “lithium nickel cobalt manganese composite oxide”) as constituent metallic elements, for example, is known as one such type of composite oxide. A lithium nickel cobalt manganese composite oxide, which has a stratified rock salt type structure generally represented by Li(Ni1/3Co1/3Mn1/3)O2, has a lattice structure in which bivalent nickel, trivalent cobalt, and tetravalent manganese are regularly arranged, and it can serve as a positive electrode active material with a high level of both structural and thermal stability.
Patent documents 1 and 2 can be noted as examples of prior art relevant to this kind of lithium nickel cobalt manganese composite oxide. These patent documents list a variety of ways for improving the characteristics of a lithium nickel cobalt manganese composite oxide that is used as the positive electrode active material in lithium secondary batteries. For example, these patent documents indicate that improvements in battery performance such as output characteristics, etc., can be obtained by using a lithium nickel cobalt manganese composite oxide wherein the atomic ratio of lithium, nickel, cobalt, and manganese is restricted to a given range as the positive electrode active material.